n The base of a tunnel on which the road or railway may be laid and used when construction is through unstable ground. It may be flat or form a continuous curve with the tunnel arch. invert (in'‑vert) The floor or bottom of the internal cross section of a closed conduit, such as an aqueduct, tunnel, or drain - The term originally referred to the inverted arch used to form the bottom of a masonry‑lined sewer or tunnel (Jackson, 1997) Wilson, W.E., Moore, J.E., (2003) Glossary of Hydrology, Berlin: Springer
n (context|civil engineering) The lowest point inside a pipe at a certain point.
n (context|civil engineering) An elevation of a pipe at a certain point along the pipe.